1. Field of the Invention
The present invention generally relates to an improvement of a variable valve timing apparatus for an internal combustion engine. More specifically, the present invention relates a variable valve timing apparatus that changes the valve timing of the intake and exhaust valves by changing the phase of the camshaft relative to that of the crankshaft.
2. Background Information
Japanese Laid-Open Patent Publications H4-272411 and H10-153105 disclose variable valve timing apparatuses for an internal combustion engine that change the valve timing of the intake and exhaust valves. The variable valve timing apparatuses disclosed by these publications are of a type that uses an electromagnetic brake.
In other words, this type of variable valve timing apparatus has a moving member in a rotational torque transmission path that transmits torque from a rotary crank body to a camshaft. The rotary crank body rotates in sync with a crankshaft. A drum that is coupled to the moving member is biased toward a rotational direction of the camshaft by a biasing means such as a torsional spring. By attracting an axial surface of the drum toward the electromagnetic brake with a magnetic force, the electromagnetic brake is attracted (biased) toward the drum. By delaying rotations of the drum and displacing the moving member in the axial direction, the phase of the camshaft relative to that of the rotary crank body is changed. This electromagnetic brake is housed within a cover that is fixed to the engine main body side while the electromagnetic brake is in a state in which rotation relative to the engine main body is restricted.
There exists a need for an improved variable valve timing apparatus for an internal combustion engine. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
It has been discovered that in variable valve timing apparatus using an electromagnetic brake, a stable braking torque cannot be generated unless surfaces of the electromagnetic brake and the drum, which axially oppose each. other, uniformly face and contact each other.
However, during the operation of the system, tension from a timing chain or timing belt is applied to a sprocket or pulley, which is a part of the rotary crank body. Due to this tension, a center axis of the drum that is coupled to the rotary crank body becomes inclined. Accordingly, the surfaces of the electromagnetic brake and the drum that are opposite each other may stop uniformly facing and contacting each other. Moreover, as a result of this inclination of the center axis of the drum, the opposing surfaces of the electromagnetic brake and the drum may partially contact each other. In such case, a desired frictional resistance cannot be obtained. Therefore, the phase adjustment will be Inegatively affected.
Alternatively, a bearing can be provided on an end of the camshaft, such that the electromagnetic brake can be rotatably supported by the camshaft via the bearing. However, the structure becomes very complicated in this case. Accordingly, this arrangement is not desirable from the point of view of cost and layout.
Also, where a certain clearance is secured between the opposing surfaces of the electromagnetic brake and the drum, such that excessive friction will not be generated between the opposing surfaces of the electromagnetic brake and the drum due to inconsistency in size of members, as axial vibrations are inputted into the electromagnetic brake, the electromagnetic brake may vibrate axially or collide into the cover or the drum, causing noises.
Furthermore, as such clearance is enlarged, the amount of power of the electromagnetic brake that is required to attract the drum becomes greater. Accordingly, the electromagnetic brake may need to be larger, power to be consumed may increase, and accordingly the fuel efficiency may be worsened.
Additionally, as such clearance becomes larger, the behavior of the electromagnetic brake becomes unstable during the operation. Accordingly, it is difficult for the electromagnetic brake to attract the surface of the drum with a stable uniform force.
The present invention has been conceived in view of the aforementioned problems. In other words, a variable valve timing apparatus for an internal combustion engine is provided that includes a rotary crank body, a moving member, a drum, a biasing member, an electromagnetic brake, a cover and a rotation regulation mechanism. The rotary crank body is adapted to be coaxially coupled with a camshaft, and to be operatively rotated by a crankshaft. The moving member operatively couples the rotary crank body to the camshaft along a rotational torque transmission path that transmits torque from the rotary crank body to the camshaft. The drum is operatively coupled to the moving member to move the moving member in an axial direction relative to the drum upon relative rotational movement between the drum and the moving member. The biasing member elastically couples the drum to the rotary crank body to bias the drum in a direction of rotation of the camshaft relative to the rotary crank body. The electromagnetic brake is disposed opposite an axial side surface of the drum to attract the axial side surface of the drum towards the electromagnetic brake with a magnetic force upon energizing the electromagnetic brake to delay rotation of the drum about the camshaft and to change a phase of the camshaft relative to a phase of the rotary crank body due to an axial movement of the moving member. The cover is fixed to an engine main body side of the electromagnetic brake to cover the electromagnetic brake. The rotation regulation mechanism is operatively coupled between a wall portion of the electromagnetic brake and a wall portion of the cover to regulate relative rotation of the electromagnetic brake about the camshaft relative to the cover. The rotation regulation mechanism includes first and second protrusions provided on one of the wall portions and first and second concavities provided on the other of the wall portions. The wall portions are arranged to oppose each other with the first and second protrusions being coupled to the first and second concavities, respectively. The first and second protrusions lying on a base line passing substantially through a center of the drum.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.